Climate change affects the geographic distribution and diversity of bogs

Finland has a lot of bogs. Climate change is unlikely to change this fact per se, but the geographic distribution of wetland complexes at least is likely to change. The loss of palsas in the zone currently characterised by permafrost will deplete the biodiversity of our peatland ecosystems, and hydrological changes may affect the ability of bogs to sequester carbon.

The many uses of bogs

The flatness of Finland's topography and the humid and cool climate favour bog formation. Originally, bogs covered approximately one third of Finland's area, and our peatland ecosystems are among the most diverse in the world and characteristic of Fennoscandia. Bogs have been used heavily in Finland, and the ploughing-up of land, peat extraction, the conversion of thin peats into mineral soils by means of drainage for forestry purposes, and hydraulic engineering and other construction have reduced the numbers of virgin bogs. Undrained peatland accounts for approximately 40% of the original area of bogs. At the moment, approximately half of all studied bog biotopes have been classified as nationally threatened [1].

Raised bogs may spread northwards

Southern bog biotopes are expected to spread northwards as a result of climate change, and raised bogs may replace some of the string bogs found in the north. Topography and rainfall nevertheless limit this development, and string bogs are unlikely to disappear altogether [2].

Goodbye to palsas

Climate change causes permafrost to melt. This is why climate change has the biggest impact on bog biotopes the structure and vegetation of which are shaped by frost phenomena. These include, in particular, pine-dominated permafrost mires as well as palsas and northern boreal string bogs which feature characteristics of pine-dominated permafrost mires [1]. The melting of ice-cored palsas and peat moss hummocks has accelerated over the last few decades, and new palsas are not formed at the same rate. An average temperature increase of four degrees is estimated to melt the palsas found in northern Fennoscandia before the end of the current century. As palsas and peat moss hummocks melt, bogs take on more and more of the characteristics of string bogs and the typical succession and diverse microhabitats of palsa bogs will be lost. This may lead to a decline of the rich birdlife of the north as well as some species of plants and invertebrate animals. The melting of palsas also increases the methane production of bogs [3] [4].

palsasuo © Aarno Torvinen

A palsa near the town of Enontekiö.

The availability of water regulates the functioning of ecosystems – will topography, vegetation, and carbon stores change?

Bogs are sensitive to hydrological changes. In Finland, air temperature is expected to rise and evaporation to increase, but annual rainfall may also increase, which is why bogs may not suffer from the effects of dryness at least to the same extent as in other parts of the world. Some degree of hydrological change is nevertheless possible especially during dry spells in the summer.

A drop in the water level in bogs would change the relative prevalence of different species of plants. In raised bogs, it would reduce the percentage of peat mosses (Sphagnum) and increase the percentage of subshrubs [5], while the wet parts of string bogs could become more barren and the prevalence of peat moss increase [2]. The topography of string bogs and raised bogs – especially the prevalence of wet parts such as puddles, flarks, and hollows – depends on rainfall. The flarks found in string bogs appear to tolerate dry periods of summer better than other areas of the bogs. Even with high evaporation levels, flarks retain their wetness as their surfaces drop and condense. If summers become drier in the future, bog biotopes characterised by periodic dryness, such as poor fens dominated by compact bog-moss (S. compactum), may also develop [6].

Bogs sequester carbon over time, which makes them important carbon stores. Peat accounts for more than two thirds of Finland's carbon stores [1]. Carbon is sequestered when organic matter decomposes slowly in anaerobic conditions while water levels are high [5]. If the bog surface dries, the decomposition of organic matter accelerates and bogs begin to release carbon [7]. On the other hand, this decreases the production of methane [8]. The effect of climate change on water levels in Finnish bogs and therefore on the role of bogs as carbon sinks is not yet clear. The lengthening of the growing season could even increase carbon sequestration in bogs [9]. However, more research on the subject is required.

References

  1. Kaakinen, E., Kokko, A., Aapala, K., Kalpio, S., Eurola, S., Haapalehto, T., Heikkilä, R., Hotanen, J.-P., Kondelin, H., Nousiainen, H., Ruuhijärvi, R., Salminen, P., Tuominen, S., Vasander, H. & Virtanen, K. 2008a. Suot. - Julkaisussa: Raunio, A., Schulman, A. & Kontula, T. (toim.). Suomen luontotyyppien uhanalaisuus. Osa 1. Tulokset ja arvioinnin perusteet. Suomen ympäristö 8/2008. S. 75–109.
  2. Laine, J., Martikainen, P., Myllys, M., Sallantaus, T., Silvola, J., Tolonen, K. & Vasander, H. 1996. Suot. Julk.: Kuusisto, E., Kauppi, L. & Heikinheimo, P. (toim.). Ilmastonmuutos ja Suomi. SILMU. Yliopistopaino, Helsinki. S. 107–126.
  3. Luoto, M., Heikkinen, R.K. & Carter, T.R. 2004. Loss of palsa mires in Europe and biological consequences. Environmental Conservation 31: 30–37.
  4. Fronzek, S., Luoto, M. & Carter, T. R. 2006. Potential effect of climate change on the distribution of palsa mires in subarctic Fennoscandia. Climate Research 32: 1–12.
  5. Riutta, T., Laine, J. & Tuittila, E.-S. 2007. Sensitivity of CO2 exchange of fen ecosystem components to water level variation. Ecosystems 10: 718-733.
  6. Ruuhijärvi, R. 2008. Ilmastonmuutoksen mahdollisia vaikutuksia suokasvillisuudessa. - Julkaisussa: Raunio, A., Schulman, A. & Kontula, T. (toim.). Suomen luontotyyppien uhanalaisuus. Osa 1. Tulokset ja arvioinnin perusteet. Suomen ympäristö 8/2008. s. 97-97.
  7. Laine, J. Soiden hiilitase ja ilmastonmuutos. Metsäntutkimuslaitos, Suoseuran 60-vuotisjuhlaseminaari 23.10.2009. http://www.suoseura.fi/fin/60/Laine.pdf
  8. Yrjälä, K., Tuomivirta, T., Juottonen, H., Putkinen, A., Lappi, K, Tuittila, E., Penttilä, T., Minkkinen, K., Laine, J., Peltoniemi, K. & Fritze, H. 2010. CH4 production and oxidation processes in a boreal fen ecosystem after long-term water table drawdown. Global Change Biology.
  9. Mäkilä, M. & Moisanen, M. 2007. Holocene lateral expansion and carbon accumulation of Luovuoma, a northern fen in Finnish Lapland. Boreas 36: 198–210.

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